Nonvolatile semiconductor memory device

ABSTRACT

A nonvolatile semiconductor memory device includes a control circuit configured to control a soft program operation of setting nonvolatile memory cells to a first threshold voltage distribution state of the nonvolatile memory cells. When a characteristic of the nonvolatile memory cells is in a first state, the control circuit executes the soft program operation by applying a first voltage for setting the nonvolatile memory cells to the first threshold voltage distribution state to first word lines, and applying a second voltage higher than the first voltage to a second word line. When the characteristic of the nonvolatile memory cells is in a second state, the control circuit executes the soft program operation by applying a third voltage equal to or lower than the first voltage to the first word lines and applying a fourth voltage lower than the second voltage to the second word line.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims the benefit of priorityunder 35 U.S.C. § 120 from U.S. Ser. No. 17/064,053, filed Oct. 6, 2020,which is a continuation of and claims the benefit of priority under 35U.S.C. § 120 from U.S. Ser. No. 16/574,637, filed Sep. 18, 2019, whichis a continuation of and claims the benefit of priority under 35 U.S.C.§ 120 from U.S. Ser. No. 16/025,429, filed Jul. 2, 2018, which is acontinuation of and claims the benefit of priority under 35 U.S.C. § 120from U.S. Ser. No. 15/706,250, filed Sep. 15, 2017, which is acontinuation of and claims the benefit of priority under 35 U.S.C. § 120from U.S. Ser. No. 15/345,585, filed Nov. 8, 2016, which is acontinuation of and claims the benefit of priority under 35 U.S.C. § 120from U.S. Ser. No. 14/945,569, filed Nov. 19, 2015 which is acontinuation of U.S. Ser. No. 14/295,923, filed Jun. 4, 2014, now U.S.Pat. No. 9,214,237, which is a continuation of U.S. Ser. No. 14/066,875,filed Oct. 30, 2013, now U.S. Pat. No. 8,787,091, which is acontinuation of U.S. Ser. No. 13/457,560, filed Apr. 27, 2012, now U.S.Pat. No. 8,599,617, which is a continuation of U.S. Ser. No. 12/878,624,filed Sep. 9, 2010, now U.S. Pat. No. 8,199,579 and claims the benefitof priority from prior Japanese Patent Application Nos. 2009-214143,filed on Sep. 16, 2009, and No. 2010-28109, filed on Feb. 10, 2010, theentire contents of each of which are incorporated herein by reference.

BACKGROUND Field

Embodiments described herein relates to a nonvolatile semiconductormemory device configured with nonvolatile memory cells that areelectrically rewritable.

Description of the Related Art

NAND type flash memory is known as nonvolatile semiconductor memorydevice that is electrically rewritable and suitable for highintegration. In NAND type flash memory, a plurality of memory cells areconnected in series in a way that the source diffused layer of onememory cell is shared as the drain diffused layer of its adjoiningmemory cell, thereby forming a NAND cell unit. Both ends of the NANDcell unit are connected to a bit line and a source line respectively viaselect gate transistors. Such a NAND cell unit configuration enableslarge-capacity storage with a smaller unit cell area than that of a NORtype memory.

A memory cell of a NAND type flash memory includes a charge accumulationlayer (floating gate electrode) provided above a semiconductor substratevia a tunnel insulating film and a control gate electrode stacked abovethe floating gate electrode via an inter-gate insulating film, andstores data in a nonvolatile manner in accordance with the chargeaccumulation state of the floating gate electrode. For example, a memorycell executes binary data storage by defining, for example, a highthreshold voltage state in which electrons are into the floating gateelectrode as data “0”, and defining a low threshold voltage state inwhich electrons in the floating gate electrode are discharged as data“1”. Recently, multi-value storage of four-value, eight-value, and so onis also undertaken by subdividing a threshold voltage distribution forwriting.

A data writing operation of a NAND type flash memory is executed on apage basis. A page is configured by memory cells arranged along aselected word line. Specifically, a writing operation is executed as anoperation of supplying a writing voltage to a selected word line andinjecting electrons into the floating gate electrode from a cell channelby the effect of FN tunneling. In this case, the potential of the cellchannel is controlled in accordance with data “0” or “1” that is to bewritten.

That is, when data “0” is to be written, a voltage Vss is supplied tothe bit line and transferred to the channel of the selected memory cellvia a select gate transistor which is conductive. At this time, in theselected memory cell, a high electrical field is applied between thefloating gate electrode and the channel to cause electrons to beinjected into the floating gate electrode. On the other hand, when data“1” is to be written (i.e., in the case of non-writing), a supplyvoltage Vdd is supplied to the bit line to charge the cell channel up toa voltage Vdd-Vth (where Vth is the threshold voltage of the select gatetransistor), after which the select gate transistor becomesnon-conductive state to turn the cell channel into a floating state. Atthis time, the potential of the cell channel rises due to the effect ofcapacitance coupling with the word line, thereby inhibiting electronsfrom being injected into the floating gate electrode.

Recently, as the minimum feature size has become increasingly smaller,the effect caused by capacitance coupling between the floating gateelectrodes of adjoining memory cells (inter-cell interference), etc. hasbecome more significant. This effect might cause undesirable dispersionof the threshold voltages of memory cells (writing error and eraseerror). Particularly, when a memory cell at the end of the NAND cellunit is directly connected to a select gate transistor, there may occurdispersion between the memory cell at the end of the NAND cell unit andthe other memory cells in regard to their operation characteristic,increasing the possibility of erroneous writing and erase error. Aneffective measure for this problem is a method of providing a dummy cellthat is not used for data storage at a location adjoining the selectgate transistor.

Further, a method of executing a so-called soft program operation isknown for solving an over-erased state of the memory cells subsequent tosimultaneous erasing. The soft program operation is important forpreventing a change of data due to capacitance coupling between thefloating gate electrodes of adjoining memory cells. Especially, thisoperation is critical as a countermeasure technique against writingerror in case of shrinking of a NAND type flash memory.

In a nonvolatile semiconductor memory such as a NAND type flash memory,etc., it is preferred to suppress dispersion of the threshold voltagesof the nonvolatile memory cells before an erasing operation is executed.Hence, it is proposed that a weak writing operation called pre-programoperation be executed before an erasing operation is executed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagram showing a memory cell array and a control circuitof a nonvolatile semiconductor memory device according to a firstembodiment.

FIG. 1B is a block diagram showing a basic configuration of a controlcircuit of a nonvolatile semiconductor memory device according to anembodiment.

FIG. 2A is a diagram showing threshold voltage distributions of thenonvolatile semiconductor memory device according to the firstembodiment.

FIG. 2B is a diagram showing threshold voltage distributions of thenonvolatile semiconductor memory device according to the firstembodiment.

FIG. 3 is a diagram explaining a soft program operation of thenonvolatile semiconductor memory device according to the firstembodiment.

FIG. 4A is a diagram explaining a problem of a soft program operation ofa nonvolatile semiconductor memory device.

FIG. 4B is a diagram explaining a problem of a soft program operation ofa nonvolatile semiconductor memory device.

FIG. 5A is a diagram explaining a problem of a soft program operation ofa nonvolatile semiconductor memory device.

FIG. 5B is a diagram explaining a problem of a soft program operation ofa nonvolatile semiconductor memory device.

FIG. 5C is a diagram explaining a problem of a soft program operation ofa nonvolatile semiconductor memory device.

FIG. 6A is a diagram explaining the soft program operation of thenonvolatile semiconductor memory device according to the firstembodiment.

FIG. 6B is a diagram explaining the soft program operation of thenonvolatile semiconductor memory device according to the firstembodiment.

FIG. 7A is a diagram explaining the soft program operation of thenonvolatile semiconductor memory device according to the firstembodiment.

FIG. 7B is a diagram explaining the soft program operation of thenonvolatile semiconductor memory device according to the firstembodiment.

FIG. 8A is a diagram explaining the soft program operation of thenonvolatile semiconductor memory device according to the firstembodiment.

FIG. 8B is a diagram explaining the soft program operation of thenonvolatile semiconductor memory device according to the firstembodiment.

FIG. 8C is a diagram explaining the soft program operation of thenonvolatile semiconductor memory device according to the firstembodiment.

FIG. 9A is a diagram explaining the soft program operation of thenonvolatile semiconductor memory device according to the firstembodiment.

FIG. 9B is a diagram explaining the soft program operation of thenonvolatile semiconductor memory device according to the firstembodiment.

FIG. 10A is a diagram explaining a soft program operation of anonvolatile semiconductor memory device according to a secondembodiment.

FIG. 10B is a diagram explaining the soft program operation of thenonvolatile semiconductor memory device according to the secondembodiment.

FIG. 11 is a flowchart explaining an operation of a control circuitaccording to an embodiment.

FIG. 12 is a flowchart explaining an operation of a control circuitaccording to an embodiment.

FIG. 13 is a flowchart explaining an operation of a control circuitaccording to an embodiment.

FIG. 14 is a flowchart explaining an operation of a control circuitaccording to an embodiment.

FIG. 15 is a flowchart showing an operation according to a thirdembodiment.

FIG. 16 is an explanatory diagram showing a concept of pre-program.

FIG. 17A is a diagram explaining pre-program according to a comparativeexample of the third embodiment.

FIG. 17B is a diagram explaining pre-program according to a comparativeexample of the third embodiment.

FIG. 18A is a diagram explaining pre-program according to the thirdembodiment.

FIG. 18B is a diagram explaining pre-program according to the thirdembodiment.

FIG. 19 is a flowchart showing an operation according to a firstmodification example of the third embodiment.

FIG. 20 is a flowchart showing an operation according to a secondmodification example of the third embodiment.

FIG. 21 is a flowchart showing an operation according to a thirdmodification example of the third embodiment.

FIG. 22 is a flowchart showing an operation according to a fourthmodification example of the third embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

A nonvolatile semiconductor memory device according to one aspect of thepresent invention includes: a memory cell array configured as anarrangement of NAND cell units each including a memory string and selecttransistors connected to both ends of the memory string respectively,the memory string including a plurality of nonvolatile memory cellsconnected in series; word lines connected to control gate electrodes ofthe nonvolatile memory cells; bit lines connected to first ends of theNAND cell units; a source line connected to second ends of the NAND cellunits; and a control circuit configured to simultaneously erase datastored in the nonvolatile memory cells arranged in a certain area, andthen control a soft program operation of setting the nonvolatile memorycells in the certain area to a first threshold voltage distributionstate of the nonvolatile memory cells. When a characteristic of thenonvolatile memory cells is determined as being in a first state, thecontrol circuit executes the soft program operation by applying a firstvoltage for setting the nonvolatile memory cells to the first thresholdvoltage distribution state to first word lines of the word lines excepta second word line connected to the nonvolatile memory cell at the endof the NAND cell unit, and applying a second voltage higher than thefirst voltage by a certain voltage value to the second word line. Whenthe characteristic of the nonvolatile memory cells is determined asbeing in a second state, the control circuit executes the soft programoperation by applying a third voltage equal to or lower than the firstvoltage to the first word lines and applying a fourth voltage lower thanthe second voltage by a certain voltage value to the second word line.

Next, the embodiments of the present invention will be explained indetail with reference to the drawings. In the attached drawings, anyportions having the same configuration are denoted by the same referencenumerals and redundant explanation thereof will be omitted indescriptions of the following embodiments. In the following embodiments,explanation will be given on the assumption that a nonvolatilesemiconductor memory device is a NAND type flash memory that uses memorycells having a stacked gate structure. However, such a configuration isa mere example, and needless to say, the present invention is notlimited to this.

First Embodiment

[Configuration of Nonvolatile Semiconductor Memory Device According toFirst Embodiment]

The configuration of a nonvolatile semiconductor memory device accordingto the first embodiment of the present invention will be explained withreference to FIG. 1A.

FIG. 1A is a diagram showing a memory cell array and a control circuitof a NAND type flash memory according to the present embodiment. A NANDcell unit 1 of the NAND type flash memory includes a source-side selectgate transistor STS and a drain-side select gate transistor STD, dummycells DC connected to the select gate transistors STS and STDrespectively, and a plurality of memory cells MCn (n=0 to 63) connectedin series between the dummy cells DC. In the NAND cell unit 1, theplurality of memory cells MCn share their source and drain regions withadjoining memory cells and form a memory string. The memory cell arrayis configured with a plurality of NAND cell units 1 that are arranged ina matrix.

The memory cell MC includes N type source/drain regions that are formedin a P type well of a silicon substrate, and has a stacked gatestructure including a control gate electrode and a floating gateelectrode as a charge accumulation layer. The NAND type flash memorychanges the amount of charge stored in the floating gate electrode by awriting operation and an erasing operation. Thereby, it changes thethreshold voltage of the memory cell MC to store one-bit or multi-bitdata in one memory cell. Here, if the memory cells MC0 and MC63 aredirectly connected to the select gate transistors STS and STD, therewill occur dispersion in the operation characteristic between the memorycells M0 and M63 located at the ends of the NAND cell unit 1 and theremaining memory cells MC. Therefore, the dummy cells DC that are notused for normal data storage are provided at the ends of the NAND cellunit 1 to obtain uniform characteristic of the memory cells MC used fordata storage.

Note that the memory cell array may be configured with no dummy cells DCprovided at the ends of the NAND cell unit 1. In this case, the memorycells MC0 and MC 63 provided at the ends of the NAND cell unit 1 arealso used for information storage. In the following embodiment,explanation will be given on the premise that the dummy cells DC areprovided at the ends of the NAND cell unit 1. However, the NAND typeflash memory according to the present invention is not limited to this.That is, the present invention can also be applied to a NAND type flashmemory in which the memory cells MC at the ends of the NAND cell unit 1are used not as dummy cells DC but as cells for data storage.

The control gate electrodes of a plurality of memory cells MCn arrangedin the X direction of FIG. 1A are commonly connected by a word line WLn(n=0 to 63). The gate electrodes of a plurality of source-side selectgate transistors STS are commonly connected by a source-side select gateline SGS. The gate electrodes of a plurality of drain-side select gatetransistors STD are commonly connected by a drain-side select gate lineSGD. The control gate electrodes of a plurality of dummy cells DCarranged in the X direction of FIG. 1A are commonly connected by adrain-side dummy word line WLDD or a source-side dummy word line WLDS.In the NAND type flash memory, an aggregate of a plurality of NAND cellunits 1 that share word lines WLn form a block.

A bit line contact BLC is connected to the drain region of thedrain-side select gate transistor STD. The bit line contact BLC isconnected to a bit line BL extending in the Y direction of FIG. 1A. Thesource-side select gate transistor STS is connected via its sourceregion to a source line SL extending in the X direction of FIG. 1A.Provided at one end of the bit lines BL is a sense amplifier circuit SA,which is used for operations of reading, writing, and erasing of celldata, as well as for operation of a soft program and a pre-program.Provided at one end of the word lines WL is a row decoder/driver 2,which selects and drives the word lines WL, the dummy word lines WLDSand WLDD, and the select gate lines SGS and SGD.

The NAND type flash memory is provided with a control circuit 3 thatcontrols a reading operation, a writing operation, and an erasingoperation to the memory cell array, and is used for a soft programoperation and a pre-program operation to be described in detail later.The control circuit 3 determines based on various information whetherthe characteristic of the dummy cells DC and memory cells MC provided inthe NAND cell unit 1 is in an initial state or a deteriorated state orto what degree the deteriorated state has advanced. For example, thecontrol circuit 3 performs this determination based on the number oftimes a pulse has been applied in an erasing operation, the number oftimes a writing/erasing operation has been executed to the NAND typeflash memory, the number of times a pulse has been applied in a writingoperation, the number of times a pulse has been applied in a softprogram operation, or the like. The control circuit 3 stores the numberof times a writing/erasing operation has been executed to the NAND typeflash memory and the number of times a pulse has been applied in awriting operation or a soft program operation based on operations of thesense amplifier circuit SA and row decoder/driver 2.

FIG. 1B is a block diagram showing a basic configuration of the controlcircuit 3. The control circuit 3 includes a pre-program unit 31, a softprogram unit 32, an erasing unit 33, and a verify unit 34. The controlcircuit 3 controls a pre-program operation, a soft program operation, anerasing operation, and various verify operations to be described later.

Next, data storing states of the NAND type flash memory according to thepresent embodiment will be explained with reference to FIG. 2A and FIG.2B. FIG. 2A and FIG. 2B are diagrams showing threshold voltagedistributions of the memory cells MC of the NAND type flash memoryaccording to the present embodiment.

When the memory cells MC of the NAND type flash memory store binary data(1 bit per cell), threshold voltage distributions of data are as shownin FIG. 2A. A state where the threshold voltage is negative representsdata “1” (erased state), and a state where the threshold voltage ispositive represents data “0”. When the memory cells MC of the NAND typeflash memory store four-value data (2 bits per cell), threshold voltagedistributions of data are as shown in FIG. 2B. In this case, four typesof threshold voltage distributions (E, A, B, and C) are set from a lowerthreshold voltage side. Four patterns of data “11,”, “01”, “00”, and“10” are assigned to these threshold voltage distributions respectively.Here, the threshold voltage distribution E is a negative thresholdvoltage state that is obtained by simultaneous block erasing.

In a data reading operation of the NAND type flash memory, a readingpass voltage Vread, which makes non-selected memory cells electricallyconductive irrespective of the data stored therein, is applied tonon-selected word lines WL in the memory cell array. Different readingpass voltages Vread may be applied to different non-selected memorycells. A reading pass voltage, which makes the dummy cells DC and theselect gate transistors STS and STD electrically conductive, is appliedto the dummy word lines WLDD and WLDS and the select gate lines SGS andSGD. Different reading pass voltages may be applied to the dummy cellsDC and the select gate transistors STS and STD.

In an operation of reading binary data, a voltage between the twothreshold voltage distributions (e.g., a voltage of 0V) is applied to aselected word line WL that is connected to a selected memory cell MC.Reading of data is executed by detecting whether or not a current flowsthrough the NAND cell unit 1 in response to the application of thevoltage. On the other hand, in an operation of reading four-value data,the voltage value of the voltage to be applied to a selected word lineWL is set according to the four threshold voltage distributions of theselected memory cell MC. That is, it is set to a voltage AR, BR, or CRbetween the respective threshold voltage distributions. The voltage ARis the lowest voltage, and voltage values increase from BR to CR. In anoperation of reading four-value data, reading of data is executed bydetecting whether or not a current flows through the NAND cell unit 1 inaccordance with which of the voltages AR, BR, and CR is applied.

In an operation of writing data “0”, a writing voltage Vpgm (e.g., 15Vto 20V) is applied to a selected word line WL. A voltage Vss is suppliedto the bit line BL and transferred to the channel (hereinafter referredto as “cell channel”) of the selected memory cell MC via the drain-sideselect gate transistor STD which is conductive. At this time, a highelectrical field is applied between the floating gate electrode and cellchannel of the selected memory cell MC to cause electrons to be injectedinto the floating gate electrode from the cell channel by the effect ofFN tunneling. When multi-value data is stored, it is possible to set aplurality of threshold voltage distributions by adjusting the amount ofelectrons to be injected into the floating gate by varying the number oftimes of writing pulse application.

In an operation of writing data “1” (non-writing), a power supplyvoltage Vdd is supplied to the bit line BL and transferred to the cellchannel of a selected memory cell MC via the drain-side select gatetransistor STD which is conductive. After the cell channel is charged upto a voltage Vdd-Vth (where Vth is the threshold voltage of the selectgate transistor), the select gate transistor becomes non-conductive andturns the cell channel into a floating state. In this case, even if awriting voltage Vpgm is applied to the selected word line WL, thepotential of the cell channel will rise due to capacitance coupling withthe selected word line WL, and electrons are not injected into thefloating gate electrode. As a result, the memory cell MC will keep thedata “1”.

A data erasing operation of the NAND type flash memory is executed on ablock basis. A data erasing operation is executed by setting all theword lines WL in a selected block including its dummy word lines WLDDand WLDS to 0V and applying a positive boosted erasing voltage (e.g.,18V to 20V) to the P-type well in which the memory cell array is formed.Consequently, all the memory cells MC in the selected block turn to anegative threshold voltage state (erased state) in which electrons inthe floating gate electrodes are discharged. After this, an erasingverify operation is executed as needed. An erasing verify operation isexecuted as an operation of checking whether or not all the memory cellsMC in the NAND cell unit 1 have been erased to a negative thresholdvoltage. Specifically, an erasing verify operation supplies a certainvoltage (e.g., 0V) to all the word lines and detects whether or not acurrent flows through the NAND cell unit 1.

As described above, an erasing operation of the NAND type flash memoryis executed simultaneously on all the memory cells in one block.Therefore, it is difficult to individually control the threshold voltageof each memory cell MC to an appropriate value. As a measure for this,the nonvolatile semiconductor memory device according to the presentembodiment suppresses dispersion of the threshold voltages of the memorycells and the dummy cells by executing a soft program operation or apre-program operation to be described later. As a result, thresholdvoltage state before a writing operation is set to an appropriate state,and the number of block defection is reduced.

In the following embodiment, controls of the soft program operation andthe pre-program operation will be explained. First, in the first andsecond embodiments, a soft program operation and control thereofexecuted by the NAND type flash memory will be explained. After this, inthe third embodiment, a pre-program operation of the NAND type flashmemory will be explained.

[Soft Program Operation]

Next, a soft program operation of the NAND type flash memory will beexplained with reference to FIG. 3. In the erasing operation describedabove, normally, the lower limit of the threshold voltage distributionof the memory cells MC is not controlled. Therefore, the thresholdvoltage distribution of the memory cells MC after being erased is like athreshold voltage distribution EBS shown in the left-hand side of FIG.3. In this case, the NAND cell unit 1 may possibly include memory cellsMC that are in an over-erased state. In case memory cells MC havedifferent threshold voltages, there may occur in a later operation achange of data (writing error) due to capacitance coupling between thefloating gate electrodes of adjoining memory cells MC. Hence, a softprogram operation that uses a weaker writing condition, i.e., a writingvoltage Vspgm (e.g., 10V to 15V) that is lower than a normal writingvoltage (e.g., 15V to 20V) is executed to all the memory cells MC andthe dummy cells DC to eliminate the over-erased state. Accordingly, thethreshold voltage distribution of the memory cells MC becomes like athreshold voltage distribution EAS shown in the right-hand side of FIG.3. As a result of the soft program operation, the range of the thresholdvoltage distribution of the memory cells MC can be made narrower on thewhole.

A soft program verify operation is executed after the soft programoperation. This operation is executed as an operation of checkingwhether the threshold voltage of a certain number of memory cells MC ordummy cells DC has exceeded a soft program verify level 1 (voltageVspv1). This operation determines a verify pass when the thresholdvoltage of a certain number of memory cells MC or dummy cells DC hasexceeded the soft program verify level 1 (voltage Vspv1) shown in FIG.3. If the threshold voltage of the memory cells MC or the dummy cells DCincreased too much in the soft program operation, it would be impossibleto distinguish between an erased state and a written state. Therefore,the soft program verify operation determines a verify fail when thethreshold voltage of a certain number of memory cells MC or dummy cellsDC has exceeded a soft program verify level 2 (voltage Vspv2).

Here, the following problem will arise if the same soft program voltageVspgm is applied to all the word lines WL and the dummy word lines WLDDand WLDS in the soft program operation. Next, this problem will beexplained with reference to FIG. 4A, FIG. 4B, FIG. 5A, FIG. 5B, and FIG.5C.

First, with reference to FIG. 4A and FIG. 4B, a case will be explainedin which the number of times a writing/erasing operation has beenexecuted to the NAND type flash memory is small and hence thecharacteristic of the dummy cells DC and memory cells MC is in aninitial state (first state). When the number of times a writing/erasingoperation has been executed is small, neither the memory cells MC northe dummy cells DC have deteriorated, and there is little difference incharacteristic between them. Here, it is assumed that the same softprogram voltage Vspgm (e.g., 10 v) is applied to all the word lines WLand the dummy word lines WLDD and WLDS in the soft program operation(see FIG. 4A). Further, a drain-side select gate voltage Vsgd (e.g.,2.5V) is applied to the drain-side select gate line SGD and asource-side select gate voltage Vsgs (e.g., 0V) is applied to thesource-side select gate line SGS.

The select gate voltages Vsgd and Vsgs are lower than the soft programvoltage Vspgm. The dummy cells DC adjoin the select gate transistors STDand STS, which are applied these select gate voltages Vsgd and Vsgs,respectively, to their gate electrodes. Therefore, in the soft programoperation, the dummy cells DC are written more slowly than the normalmemory cells MC, because each of the normal memory cells MC issandwiched between the word lines WL applied with the soft programvoltage Vspgm. As a result, after the soft program operation, the dummycells DC and the normal memory cells MC have different threshold voltagevalues. That is, as shown by solid lines (After SPROG) in FIG. 4B, thethreshold voltage of the dummy cells DC becomes lower than the thresholdvoltage of the normal memory cells MC.

If a reading operation is executed plural times in this state, thethreshold voltages of the memory cells MC and dummy cells DC willincrease due to a read disturb (see a broken line (After Read Disturb)in FIG. 4B). At this time, the dummy cells DC are more largelyinfluenced by the read disturb because their threshold voltage is lower,and the amount of increase of the threshold voltage of the dummy cellsDC is larger than the amount of increase of the threshold voltage of thenormal memory cells MC. If the amount of increase of the thresholdvoltage of the dummy cells DC is larger when the read disturb occurs, aninter-cell interference occurs between the dummy cells DC and theiradjoining memory cell MC0 or MC63 to increase the threshold voltage ofthe memory cells MC0 and MC63. As a result, there arises a problem thatan unintentional change (writing error) occurs in the data of the memorycells MC0 and MC63.

Next, with reference to FIG. 5A, FIG. 5B, and FIG. 5C, a case will beexplained in which a writing/erasing operation has been repeated to theNAND type flash memory plural times and hence the dummy cells DC and thememory cells MC are in a deteriorated state (second state). Throughplural times of repetitive writing/erasing operations to the memorycells MC, the dummy cells DC in the vicinity of the ends of the NANDcell unit 1 have their tunnel insulating film deteriorated faster thanthe normal memory cells MC in the NAND cell unit 1 due to the influenceof the select gate transistors STS and STD. It is assumed that thetunnel insulating film of the dummy cells DC is deteriorated. In casethe same soft program operation would cause electrons to be injectedinto the floating gate electrode of the dummy cells DC and the normalmemory cells MC, a writing speed of the dummy cells DC is faster thanthat of the normal memory cells MC.

Also in this soft program operation, it is assumed that the same softprogram voltage Vspgm is applied to all the word lines WIL and the dummyword lines WLDD and WLDS, as shown in FIG. 5C. When the dummy cells DCare deteriorated, they are written at a higher writing speed than thenormal memory cells MC. Therefore, as shown in FIG. 5A, the thresholdvoltage of the dummy cells DC becomes higher than the threshold voltageof the normal memory cells MC. As a result, in the threshold voltagedistribution after the soft program operation, some dummy cells DC arein an over-written state (OP: over-programmed state), as shown in FIG.5B. If there are more than a certain number of dummy cells DC that arein the over-written state OP in which their threshold voltage exceedsthe soft program verify level 2 (voltage Vspv2), the soft programoperation cannot be completed normally. Therefore, there will arise aproblem that a block that includes the NAND cell unit 1 in question maybe determined as an error block. Furthermore, after a soft-programoperation is executed, if the threshold voltages of the dummy cells aregreatly different from the threshold voltages of the memory cells, itwill become impossible to converge the threshold voltages of the dummycells and the memory cells within a certain range in a writing operationto be executed later. As a result, a block that includes the dummy cellsof which the threshold voltages are greatly different from the thresholdvoltages of the memory cells may be determined as an error block.

To solve these problems, the nonvolatile semiconductor memory deviceaccording to the present embodiment executes the following soft programoperation.

[Operation of Nonvolatile Semiconductor Memory Device According to FirstEmbodiment]

FIG. 6A, FIG. 6B, FIG. 7A, and FIG. 7B are diagrams explaining the softprogram operation of the NAND type flash memory according to the presentembodiment. FIG. 6A and FIG. 6B show the soft program operation for acase where the number of times a writing/erasing operation has beenexecuted to the NAND type flash memory is small and hence thecharacteristic of the dummy cells DC and memory cells MC is in theinitial state. FIG. 7A and FIG. 7B show the soft program operation for acase where a writing/erasing operation has been repeated plural times tothe NAND type flash memory and hence the dummy cells DC and the memorycells MC are in the deteriorated state.

First, the case where the number of times a writing/erasing operationhas been executed to the NAND type flash memory is small and hence thecharacteristic of the dummy cells DC and memory cells MC is in theinitial state will be explained with reference to FIG. 6A and FIG. 6B.The NAND type flash memory according to the present embodiment candetermine whether the characteristic of the dummy cells DC and memorycells MC is in the initial state or the deteriorated state by means ofthe control circuit 3. The operation of the control circuit 3 will bedescribed in detail later. When the control circuit 3 determines thatthe characteristic of the dummy cells DC and memory cells MC is in theinitial state, the soft program voltage Vspgm is applied to the wordlines WL connected to the normal memory cells MC as shown in FIG. 6A.The soft program voltage Vspgm is set to, for example, 10V. A dummy wordline soft program voltage Vwld_spgm1 is applied to the dummy word linesWLDD and WLDS connected to the dummy cells DC. When the characteristicof the dummy cells DC and memory cells MC is in the initial state, thedummy word line soft program voltage Vwld_spgm1 is set to a voltagevalue (e.g., 11V) higher than the soft program voltage Vspgm (e.g., 10V)by a certain value. A drain-side select gate voltage Vsgd (e.g., 2.5V)is applied to the drain-side select gate line SGD, and a source-sideselect gate voltage Vsgs (e.g., 0V) is applied to the source-side selectgate line SGS.

The voltage values presented above are mere examples in the soft programoperation, and it is only necessary that the relationship in levelbetween the voltage values of the voltages applied to the word lines WLand the dummy word lines WLDD and WLDS should be such that the voltageVwld_spgm1 is higher than the voltage Vspgm. When the characteristic ofthe dummy cells DC and memory cells MC is in the initial state, the NANDtype flash memory according to the present embodiment executes the softprogram operation by applying the aforementioned voltage to the wordlines WL. The soft program operation may be an operation of repeatingapplying the soft program voltages to the word lines WL and the dummyword lines WLDD and WLDS plural times.

Here, since the select gate voltages Vsgd and Vsgs are lower than thesoft program voltage Vspgm, the speed at which the dummy cells DCadjoining the select gate transistors STD and STS are written may becomeslow. However, applied to the dummy word lines WLDD and WLDS is thedummy word line soft program voltage Vwld_spgm1, which is higher thanthe voltage Vspgm. Therefore, the speed at which the dummy cells DC arewritten becomes nearly equal to the speed at which the normal memorycells MC are written. As a result, there will not be dispersion betweenthe threshold voltage value of the dummy cells DC and that of the normalmemory cells MC after the soft program operation. That is, as shown by asolid line (After SPROG) in FIG. 6B, the threshold voltage of the dummycells DC and that of the normal memory cells MC will be substantiallythe same value after the soft program operation.

Next, the case where a writing/erasing operation has been repeatedplural times to the NAND type flash memory and hence the dummy cells DCand the memory cells MC are in the deteriorated state will be explainedwith reference to FIG. 7A and FIG. 7B. Also in this case where thecontrol circuit 3 determines that the characteristic of the dummy cellsDC and memory cells MC is in the deteriorated state, the same softprogram voltage Vspgm (e.g., 10V) is applied to the word lines WLconnected to the normal memory cells MC, as shown in FIG. 7B. Meanwhile,a dummy word line soft program voltage Vwld_spgm2 is applied to thedummy word lines WLDD and WLDS connected to the dummy cells DC. When thedummy cells DC are in the deteriorated state, the dummy word line softprogram voltage Vwld_spgm2 is set to a voltage value (e.g., 9V) that islower than the soft program voltage Vspgm by a certain value. Adrain-side select gate voltage Vsgd (e.g., 2.5V) is applied to thedrain-side select gate line SGD, and a source-side select gate voltageVsgs (e.g., 0V) is applied to the source-side select gate line SGS.

The voltage values presented above are mere examples in the soft programoperation, and it is only necessary that the relationship in levelbetween the voltage values of the voltages applied to the word lines WLand the dummy word lines WLDD and WLDS should be such that the voltageVwld_spgm2 is lower than the voltage Vspgm. When the characteristic ofthe dummy cells DC and memory cells MC is in the deteriorated state, theNAND type flash memory according to the present embodiment executes thesoft program operation by applying the aforementioned voltage to theword lines WL. The soft program operation may be an operation ofrepeating applying the soft program voltages to the word lines WL andthe dummy word lines WLDD and WLDS plural times.

The embodiment described above has explained the states of the dummycells DC as including two states, namely the initial state and thedeteriorated state. The states may be divided into three or moreplurality of states according to the degree of advancement ofdeterioration. A case where the states of the dummy cells DC include aplurality of states will be explained with reference to FIG. 8A, FIG.8B, FIG. 8C, FIG. 9A, and FIG. 9B. FIG. 8A and FIG. 8B show a case wherethe characteristic of the dummy cells DC is in the initial state. FIG.8A is a diagram showing the voltages to be applied to the word lines WLand the dummy word lines WLDD and WLDS in the soft program operation.FIG. 8B is a diagram showing the value of the threshold voltage of thememory cells after the soft program operation. FIG. 9A and FIG. 9B showa case where the characteristic of the dummy cells DC is in thedeteriorated state. FIG. 9A is a diagram showing the voltages to beapplied to the word lines WL and the dummy word lines WLDD and WLDS inthe soft program operation. FIG. 9B is a diagram showing the value ofthe threshold voltage of the memory cells after the soft programoperation.

The state shown in FIG. 8C represents a case where a writing/erasingoperation to the NAND type flash memory has been repeated plural timesand hence the dummy cells DC are in a slightly deteriorated state (thirdstate). The state that the dummy cells DC are slightly deteriorated (orthe third state) means an intermediate state that will appear when thedummy cells DC shift from the initial state to the deteriorated state.In the slightly deteriorated state (third state) of the dummy cells DC,the relationship in level between the voltage values of the voltages tobe applied to the word lines WL and the dummy word lines WLDD and WLDScan be set such that the voltage Vwld_spgm is equal to the voltageVspgm. In this case, the voltage to be applied to the dummy word linesWLDD and WLDS is referred to as voltage Vwld_spgm3.

The voltage to be applied to the dummy word lines WLDD and WLDS in thesoft program operation changes from the voltage Vwld_spgm1 to thevoltage Vwld_spgm3 and to the voltage Vwld_spgm2 as deterioration of thedummy cells DC advances (FIG. 8A, FIG. 8C, FIG. 9A). Based on theadvancement of deterioration of the dummy cells DC and memory cells MC,the relationships between the voltage Vspgm applied to the word lines WLand the voltage applied to the dummy word lines WLDD and WLDS in thesoft program operation are as follows.

Voltage Vwld_spgm1>Voltage Vspgm

Voltage Vwld_spgm3=Voltage Vspgm

Voltage Vwld_spgm2<Voltage Vspgm

In other words, the voltage applied to the dummy word line WLDD in thesoft program operation changes in a way of Voltage Vwld_spgm1>VoltageVwld_spgm3>Voltage Vwld_spgm2 as a writing/erasing operation to the NANDtype flash memory is repeated more times.

As described above, the dummy cells DC which are deteriorated byrepetition of writing/erasing operations to the NAND type flash memorywill be written at a higher speed than that of the normal memory cellsMC. However, in the soft program operation according to the presentembodiment, in the deteriorated state of the dummy cells DC, the dummyword line soft program voltage Vwld_spgm2 is set to a voltage lower thanthe soft program voltage Vspgm as shown in FIG. 7B. Further, in theslightly deteriorated state of the dummy cells DC, the dummy word linesoft program voltage Vwld_spgm3 is set to a voltage nearly equal to thesoft program voltage Vspgm as shown in FIG. 8C. Therefore, the speed atwhich the dummy cells DC are written is nearly equal to the speed atwhich the normal memory cells MC are written. As a result, the dummycells Dc and the normal memory cells MC will not have differentthreshold voltage values after the soft program operation. As shown inFIG. 7A, the threshold voltage distribution after the soft programoperation will include no cells that are in an over-written state (OP:over-programmed state).

[Effect of Nonvolatile Semiconductor Memory Device According to FirstEmbodiment]

An effect of the soft program operation of the NAND type flash memoryaccording to the present embodiment will be explained with reference toFIG. 8A to FIG. 9B.

In the NAND type flash memory according to the present embodiment,different voltages are applied to the word lines WL and the dummy wordlines WLDS and WLDD in the soft program operation, as shown in FIG. 8Aand FIG. 9A. That is, the soft program voltage Vspgm is applied to theword lines WL and the dummy word line soft program voltages Vwld_spgm 1and Vwld_spgm2 are applied to the dummy word lines WLDS and WLDD.Further, the relationship in voltage level between the soft programvoltage Vspgm and the dummy word line soft program voltage Vwld_spgm1 orVwld_spgm2 is changed depending on whether the characteristic of thedummy cells DC and memory cells MC is in the initial state or the dummycells DC are in the deteriorated state. As a result, the NAND type flashmemory according to the present embodiment can suppress dispersion inthreshold voltage between the memory cells MC and the dummy cells DCafter the soft program operation, as shown in FIG. 8B and FIG. 9B.

When the characteristic of the dummy cells DC and memory cells MC is inthe initial state, the threshold voltage of the dummy cells DC and thatof the normal memory cells MC will be substantially equal after the softprogram operation, as shown in FIG. 8B. When a reading operation isexecuted in this state, the threshold voltage of the memory cells MC anddummy cells DC will increase due to a read disturb (see a broken line(After Read Disturb) in FIG. 8B). Here, the amount of increase of thethreshold voltage of the dummy cells DC due to the read disturb issubstantially equal to the amount of increase of the threshold voltageof the normal memory cells MC because the threshold voltage of the dummycells DC is substantially equal to that of the normal memory cells MC.Therefore, when a read disturb occurs, the influence given on the memorycells MC0 and MC63 adjoining the dummy cells DC will be less, making itpossible to prevent writing error in the memory cells M0 and M63.

Also when the dummy cells DC and the memory cells MC are in thedeteriorated state, the threshold voltage of the dummy cells DC and thatof the normal memory cells MC will be substantially equal after the softprogram operation, as shown in FIG. 9B. That is, in the thresholdvoltage distribution after the soft program operation, there are notcells in an over-written state (OP: over-programmed state) (see thethreshold voltage distribution of FIG. 7A). As a result, the number ofdummy cells DC and memory cells MC that are in the over-written state OPwith a threshold voltage exceeding the soft program verify level 2(voltage Vspv2) will not exceed the certain number, enabling the softprogram operation to be completed normally. In this way, the NAND typeflash memory according to the present embodiment can securely executethe soft program operation.

When the dummy cells DC and the memory cells MC are in the deterioratedstate, it is also conceivable to set the dummy word line soft programvoltage Vwld_spgm2 equal to the soft program voltage Vspgm and apply thedummy word line soft program voltage Vwld_spgm 2 to the dummy word linesa smaller number of times than the soft program voltage Vspgm is appliedto the word lines. However, this scheme cannot manage a case where thethreshold of the dummy cells DC will exceed the soft program verifylevel 2 (voltage Vspv2) when the dummy word line soft program voltageVwld_spgm2 (=Vspgm) is applied to the dummy word lines only once.

Second Embodiment

[Configuration of Nonvolatile Semiconductor Memory Device According toSecond Embodiment]

Next, a nonvolatile semiconductor memory device according to the secondembodiment of the present invention will be explained. The configurationof the nonvolatile semiconductor memory device according to the secondembodiment is substantially the same as the first embodiment, and willnot therefore be explained repeatedly. The nonvolatile semiconductormemory device according to the second embodiment is different from thefirst embodiment in that the voltage value of the soft program voltageVspgm applied to the word lines WL is changed.

[Operation of Nonvolatile Semiconductor Memory Device According toSecond Embodiment]

FIG. 10A and FIG. 10B are diagrams explaining a soft program operationof a NAND type flash memory according to the present embodiment. FIG.10A shows a state of voltage application to the word lines WL and thedummy word lines WLDD and WLDS in the soft program operation when thecharacteristic of the dummy cells DC and memory cells MC is in aninitial state. FIG. 10B shows a state of voltage application to the wordlines WL and the dummy word lines WLDD and WLDS in the soft programoperation when the dummy cells DC and the memory cells MC are in adeteriorated state. In the soft program operation according to thepresent embodiment, the voltage value of the soft program voltage Vspgmto be applied to the word lines WL is changed according to whether thecharacteristic of the dummy cells DC and memory cells MC is in theinitial state or in the deteriorated state.

First, with reference to FIG. 10A, the case that the number of times awriting/erasing operation has been executed to the NAND type flashmemory is small and hence the characteristic of the dummy cells DC andmemory cells MC is in the initial state will be explained. When thecontrol circuit 3 determines that the characteristic of the dummy cellsDC and memory cells MC is in the initial state, a soft program voltageVspgm is applied to the word lines WL connected to the normal memorycells MC, as shown in FIG. 10A. This soft program voltage Vspgm is setto, for example, 10V. A dummy word line soft program voltage Vwld_spgm1is applied to the dummy word lines WLDD and WLDS connected to the dummycells DC. When the characteristic of the dummy cells DC and memory cellsMC is in the initial state, this dummy word line soft program voltageVwld_spgm1 is set to a voltage value (e.g., 11V) higher than the softprogram voltage Vspgm by a certain value. A drain-side select gatevoltage Vsgd (e.g., 2.5V) is applied to the drain-side select gate lineSGD, and a source-side select gate voltage Vsgs (e.g., 0V) is applied tothe source-side select gate line SGS.

The voltage values presented above are mere examples in the soft programoperation, and it is only necessary that the relationship in voltagelevel between the voltages applied to the word lines WL and the dummyword lines WLDD or WLDS should be such that the voltage Vwld_spgm1 ishigher than the voltage Vspgm. When the characteristic of the dummycells DC and memory cells MC is in the initial state, the NAND typeflash memory according to the present embodiment executes the softprogram operation by applying the aforementioned voltage to the wordlines WL.

Next, with reference to FIG. 10B, the case that a writing/erasingoperation to the NAND type flash memory has been repeated plural timesand hence the dummy cells DC and the memory cells MC have deterioratedwill be explained. When the control circuit 3 determines that thecharacteristic of the dummy cells DC and memory cells MC is in thedeteriorated state, a soft program voltage Vspgm′ is applied to the wordlines WL connected to the normal memory cells MC, as shown in FIG. 10B.In the soft program operation according to the present embodiment, thevoltage value of the soft program voltage to be applied to the wordlines WL is changed according to whether the dummy cells DC and thememory cells MC are in the initial state or in the deteriorated state.That is, when the characteristic of the dummy cells DC and memory cellsMC is in the deteriorated state, the voltage value of the soft programvoltage Vspgm′ is set to a voltage value (e.g., 9V) lower than the softprogram voltage Vspgm in the initial state by a certain value.

A dummy word line soft program voltage Vwld_spgm2 is applied to thedummy word lines WLDD and WLDS connected to the dummy cells DC. When thedummy cells DC and the memory cells MC are in the deteriorated state,this dummy word line soft program voltage Vwld_spgm2 is set to a voltagevalue (e.g., 8V) lower than the soft program voltage Vspgm′ by a certainvalue. A drain-side select gate voltage Vsgd (e.g., 2.5V) is applied tothe drain-side select gate line SGD, and a source-side select gatevoltage Vsgs (e.g., 0V) is applied to the source-side select gate lineSGS.

The voltage values presented above are mere examples in the soft programoperation, and it is only necessary that the relationship in voltagelevel between the voltages to be applied to the word lines WL and thedummy word lines WLDD or WLDS should be such that the voltage Vwld_spgm2is lower than the voltage Vspgm′. When the characteristic of the dummycells DC and memory cells MC is in the deteriorated state, the NAND typeflash memory according to the present embodiment executes the softprogram operation by applying the aforementioned voltage to the wordlines WL. The voltage Vwld_spgm2 is lower than the voltage Vwld_spgm1,and the voltage Vspgm′ is lower than the voltage Vspgm.

[Effect of Nonvolatile Semiconductor Memory Device According to SecondEmbodiment]

Also the NAND type flash memory according to the present embodimentapplies different voltages to the word lines WL and the dummy word linesWLDS and WLDD in the soft program operation. Furthermore, therelationship in voltage level between the soft program voltage Vspgm andthe dummy word line soft program voltages Vwld_spgm1 or Vwld_spgm2 ischanged depending on whether the characteristic of the dummy cells DCand memory cells MC is in the initial state or the dummy cells DC andthe memory cells MC are in the deteriorated state. As a result, also theNAND type flash memory according to the present embodiment can suppressdispersion between the threshold voltage of the memory cells MC and thatof the dummy cells DC after the soft program operation.

When writing/erasing operations of the NAND type flash memory arerepeated plural times, not only the dummy cells DC but also the normalmemory cells MC may deteriorate. Specifically, also the memory cells MChave their tunnel insulating film deteriorated through repetition ofwriting/erasing operations, and a writing speed of the normal memorycells MC is faster. If the voltage value of the soft program voltageVspgm remains the same regardless of whether the characteristic of thememory cells MC is in the initial state or in the deteriorated state,some of the memory cells MC might become an over-written state (OP:over-programmed state) in the soft program operation. If the number ofmemory cells MC that are in the over-written state OP with a thresholdvoltage exceeding the soft program verify level 2 (voltage Vspv2)exceeds a certain number, the soft program operation cannot be completednormally.

However, in the NAND type flash memory according to the presentembodiment, the soft program voltage Vspgm′ applied to the word lines isset to a voltage lower than the soft program voltage Vspgm, as shown inFIG. 10A and FIG. 10B. Accordingly, the speed at which the memory cellsMC in the deteriorated state are written can be slowed down and therewill not occur cells that will be in the over-written state (OP:over-programmed state). As a result, the number of dummy cells DC andmemory cells MC in the over-written state OP with a threshold voltageexceeding the soft program verify level 2 (voltage Vspv2) will notexceed the certain number, enabling the soft program operation to becompleted normally. In this way, the NAND type flash memory according tothe present embodiment can securely execute the soft program operation.

Similarly to the first embodiment, there may exist a state (third state)in which the dummy cells DC are slightly deteriorated due to pluraltimes of repetitive writing/erasing operations of the NAND type flashmemory. Also in this case, the dummy word line soft program voltagechanges in a way of Voltage Vwld_spgm1>Voltage Vwld_spgm3>VoltageVwld_spgm2, as in the first embodiment.

The NAND type flash memory according to the embodiments of the presentinvention has been explained. This NAND type flash memory changes therelationship in voltage level between the soft program voltage Vspgmapplied to the word lines and the dummy word line soft program voltageVwld_spgm1 or Vwld_spgm2 according to whether the characteristic of thedummy cells DC and memory cells MC is in the initial state or in thedeteriorated state. The determination of whether the characteristic ofthe dummy cells DC and memory cells MC is in the initial state or in thedeteriorated state is performed by the control circuit 3. In thefollowing description, the operation of the control circuit 3 fordetermining the characteristic of the dummy cells DC and memory cells MCwill be explained. The determination operation of the control circuit 3to be explained below can be applied to both of the first and secondembodiments.

[Determination Operation 1 by Control Circuit 3]

FIG. 11 is a flowchart explaining the operation of the control circuit 3in the soft program operation of the NAND type flash memory according tothe present embodiment.

The soft program operation of the NAND type flash memory according tothe first and second embodiments is executed subsequently to an erasingoperation. In erasing data, as described above, the control gate voltageof the memory cells MC is set to 0V, and an erasing pulse having a highvoltage is supplied to the well in which the memory cells MC aredisposed. This causes electrons to be discharged from the floating gateelectrode to the semiconductor substrate via the tunnel insulating film,and causes the threshold voltage of the memory cells MC to shift to thenegative direction. Application of the erasing pulse is carried outplural times by incrementing its voltage value. The control circuit 3can determine the characteristic of the dummy cells DC and memory cellsMC based on the number of times the erasing pulse has been applied. Thiswill be explained below with reference to FIG. 11.

In an erasing operation of the NAND type flash memory, the erasing pulsevoltage to be applied to the well is applied plural times byincrementing the voltage value by a certain voltage value (step S11).After this, a verify operation for checking whether erasing has beencompleted is executed to confirm that the erasing operation has beencompleted. At this time, the control circuit 3 senses how many times theerasing pulse has been applied (step S12). The control circuit 3compares the number of times the erasing pulse has been applied in theerasing operation with a preset determination value (step S13).

Each time an operation is executed on the NAND type flash memory, thetunnel insulating film of the dummy cells DC and the memory cells MCdeteriorates. As the tunnel insulating film of the dummy cells DC andmemory cells MC becomes more deteriorated, the speed of an erasingoperation of discharging electrons from the floating gate electrodebecomes slower. Therefore, as the dummy cells DC and the memory cells MCbecome more deteriorated, the number of times of applying a pulsebecomes larger at an erasing operation. The control circuit 3 comparesthis number of times the erasing pulse has been applied in the erasingoperation with the determination value.

The control circuit 3 determines the state of the dummy cells DC andmemory cells MC based on the number of times the erasing pulse has beenapplied in the erasing operation and the determination value (step S14).For example, when the number of times the erasing pulse has been appliedin the erasing operation exceeds the certain value, the control circuit3 determines that the dummy cells DC and the memory cells MC are in thedeteriorated state. On the other hand, when the number of times theerasing pulse has been applied in the erasing operation is equal to orsmaller than the certain value, the control circuit 3 determines thatthe dummy cells DC and the memory cells MC are in the initial state. Thecontrol circuit 3 may have a plurality of determination values anddistinguish among a plurality of states such as an initial state, aslightly deteriorated state, a deteriorated state, etc. based on thecomparison with these determination values.

The sense amplifier circuit SA and the row decoder/driver 2 execute thevoltage applying method according to the above-described embodimentsbased on the determination result of the control circuit 3. In thedetermination operation based on the number of times a reset pulse hasbeen applied, because the number of times the erasing pulse has beenapplied is directly sensed in the erasing operation, the control circuit3 needs not store the number of times the erasing pulse has beenapplied. In this case, there is no need of securing an informationstorage area in the control circuit 3, and the NAND type flash memorycan therefore be disposed in a simpler configuration.

[Determination Operation 2 by Control Circuit 3]

FIG. 12 is a flowchart explaining the operation of the control circuit 3in the soft program operation of the NAND type flash memory according tothe present embodiment. FIG. 12 shows a case that the control circuit 3executes the determination operation based on the number of times awriting/erasing operation has been executed on the NAND type flashmemory.

In this case, each time a writing/erasing operation is executed on theNAND type flash memory, information indicating that a writing/erasingoperation has been executed is sent by the sense amplifier circuit SAand the row decoder/driver 2 into the control circuit 3. The controlcircuit 3 stores the number of times a writing/erasing operation hasbeen executed on the NAND type flash memory based on this information.The control circuit 3 can determine the characteristic of the dummycells DC and memory cells MC based on the number of times awriting/erasing operation has been executed on the NAND type flashmemory. This will be explained below with reference to FIG. 12.

When the determination operation by the control circuit 3 is started,the control circuit 3 acquires the information about the number of timesa writing/erasing operation has been executed on the NAND type flashmemory from the storage area inside the control circuit 3 (step S21).The information about the number of times a writing/erasing operationhas been executed indicates how many times a writing/erasing operationhas been executed in the past on the block to which the soft programoperation is to be executed. The control circuit 3 compares the numberof times a writing/erasing operation has been executed on the NAND typeflash memory with a preset determination value (step S22).

As described above, each time a writing/erasing operation is executed onthe NAND type flash memory, the tunnel insulating film of the dummycells DC and memory cells MC deteriorates. The control circuit 3determines the state of the dummy cells DC and memory cells MC based onthe number of times a writing/erasing operation has been executed on theNAND type flash memory and the determination value (step S23).

For example, when the number of times a writing/erasing operation hasbeen executed on the NAND type flash memory exceeds the certain value,the control circuit 3 determines that the dummy cells DC and the memorycells MC are in the deteriorated state. On the other hand, when thenumber of times a writing/erasing operation has been executed on theNAND type flash memory is equal to or smaller than the certain value,the control circuit 3 determines that the dummy cells DC and the memorycells MC are in the initial state. The control circuit 3 may have aplurality of determination values and distinguish among a plurality ofstates such as an initial state, a slightly deteriorated state, adeteriorated state, etc. based on comparison with these determinationvalues. The sense amplifier circuit SA and the row decoder/driver 2execute the voltage applying method according to the above-describedembodiments based on the determination result of the control circuit 3.

[Determination Operation 3 by Control Circuit 3]

FIG. 13 is a flowchart explaining the operation of the control circuit 3in the soft program operation of the NAND type flash memory according tothe present embodiment. FIG. 13 shows a case that the control circuit 3executes the determination operation based on the number of times apulse has been applied in a writing operation on the NAND type flashmemory.

In a data writing operation, a writing voltage (e.g., 15V to 20V) isapplied to the selected word line WL. A voltage Vss is applied to thechannel of the selected memory cell MC. This causes a high electricalfield to be applied between the floating gate electrode and cell channelof the selected memory cell MC and electrons to be injected into thefloating gate electrode from the cell channel. This writing pulse isapplied plural times by incrementing the voltage value thereof. In thewriting operation, information indicating how many times the writingpulse has been applied is sent by the sense amplifier circuit SA and therow decoder/driver 2 to the control circuit 3. The control circuit 3stores the number of times the pulse has been applied in the writingoperation based on this information. The control circuit 3 can determinethe characteristic of the dummy cells DC and memory cells MC based onthe number of times the pulse has been applied in the writing operation.This will be explained below with reference to FIG. 13.

When the determination operation by the control circuit 3 is started,the control circuit 3 acquires the information about the number of timesa pulse has been applied in a writing operation from the storage areainside the control circuit 3 (step S31). The information about thenumber of times a pulse has been applied in a writing operationindicates the number of times a pulse has been applied in the lastwriting operation on the block to which the soft program operation is tobe executed. The control circuit 3 compares the number of times a pulsehas been applied in the writing operation with a preset determinationvalue (step S32).

As described above, as the tunnel insulating film of the dummy cells DCand memory cells MC becomes more deteriorated, the speed of a writingoperation of injecting electrons into the floating gate electrodebecomes faster. Therefore, as the dummy cells DC and the memory cells MCbecome more deteriorated, the number of times of applying a pulsebecomes smaller at a writing operation. The control circuit 3 determinesthe state of the dummy cells DC and memory cells MC based on this numberof times a pulse has been applied in the writing operation and thedetermination value (step S33).

For example, when the number of times a pulse has been applied in thewriting operation is smaller than a certain value, the control circuit 3determines that the dummy cells DC and the memory cells MC are in thedeteriorated state. On the other hand, when the number of times a pulsehas been applied in the writing operation is equal to or larger than thecertain value, the control circuit 3 determines that the dummy cells DCand the memory cells MC are in the initial state. The control circuit 3may have a plurality of determination values and distinguish among aplurality of states such as an initial state, a slightly deterioratedstate, a deteriorated state, etc. based on comparison with thesedetermination values. The sense amplifier circuit SA and the rowdecoder/driver 2 execute the voltage applying method according to theabove-described embodiments based on the determination result of thecontrol circuit 3.

[Determination Operation 4 by Control Circuit 3]

FIG. 14 is a flowchart explaining the operation of the control circuit 3in the soft program operation of the NAND type flash memory according tothe present embodiment. FIG. 14 shows a case that the control circuit 3executes the determination operation based on the number of times apulse has been applied in the soft program operation on the NAND typeflash memory.

In the soft program operation, a pulse is applied plural times byincrementing the voltage value of the soft program voltage, like in thewriting operation. In this soft program operation, informationindicating how many times a soft program pulse has been applied is sentby the sense amplifier circuit SA and the row decoder/driver 2 to thecontrol circuit 3. The control circuit 3 stores the number of times apulse has been applied in the soft program operation based on thisinformation. For example, the control circuit 3 can determine thecharacteristic of the dummy cells DC and memory cells MC based on thenumber of times a pulse has been applied in the soft program operation.This will be explained below with reference to FIG. 14.

When the determination operation by the control circuit 3 is started,the control circuit 3 acquires the information about the number of timesa pulse has been applied in the soft program operation from the storagearea inside the control circuit 3 (step S41). The information about thenumber of times a pulse has been applied in the soft program operationindicates the number of times a pulse has been applied in the last softprogram operation on the block to which the soft program operation is tobe executed. The control circuit 3 compares the number of times a pulsehas been applied in the soft program operation with a presetdetermination value (step S42).

As described above, as the tunnel insulating film of the dummy cells DCand memory cells MC becomes more deteriorated, the speed of the writingoperation of injecting electrons into the floating gate electrodebecomes faster. Therefore, as the dummy cells DC and the memory cell MCbecome more deteriorated, the number of times of applying a pulsebecomes smaller at the soft program operation. The control circuit 3determines the state of the dummy cells DC and memory cells MC based onthis number of times a pulse has been applied in the soft programoperation and the determination value (step S43).

When the number of times a pulse has been applied in the soft programoperation is smaller than a certain value, the control circuit 3determines that the dummy cells DC and the memory cells MC are in thedeteriorated state. On the other hand, when the number of times a pulsehas been applied in the soft program operation is equal to or greaterthan the certain value, the control circuit 3 determines that the dummycells DC and the memory cells MC are in the initial state. The controlcircuit 3 may have a plurality of determination values and distinguishamong a plurality of states such as an initial state, a slightlydeteriorated state, a deteriorated state, etc. based on comparison withthese determination values. The sense amplifier circuit SA and the rowdecoder/driver 2 execute the voltage applying method according to theabove-described embodiments based on the determination result of thecontrol circuit 3.

The operations of the control circuit 3 for determining thecharacteristic of the cells have been explained. In the NAND type flashmemory, the determination operation of the control circuit 3 may be anyone of the operations described above or may be a combination of pluralones of them. By the way, the preset determination value may be storedin a ROM fuse in soft program circuit 32.

The operations of the embodiments described above have been explained asoperations of varying the voltages to be applied to the word lines WLnand to the dummy word lines WLDS and WLDD in the soft program operation.This operation of varying the voltages to be applied to the word linesWLn and to the dummy word lines WLDS and WLDD can also be used forcontrolling a pre-program operation executed before an erasingoperation. As a third embodiment, a pre-program operation and a controlthereof performed by the NAND type flash memory will be explained below.

Third Embodiment

[Configuration of Nonvolatile Semiconductor Memory Device According toThird Embodiment]

The nonvolatile semiconductor memory device according to the thirdembodiment of the present invention will be explained. The configurationof the nonvolatile semiconductor memory device according to the thirdembodiment is the same as the first embodiment, and hence an explanationthereof will not be provided. The nonvolatile semiconductor memorydevice according to the third embodiment is different from the firstembodiment in using the operation of varying the voltages to be appliedto the word lines WLn and to the dummy word lines WLDS and WLDD in orderto control a pre-program operation executed before an erasing operation.

[Operation of Nonvolatile Semiconductor Memory Device According to ThirdEmbodiment]

FIG. 15 is a flowchart showing the operation according to the presentembodiment. First, a pre-program operation is performed (step S11). Thatis, before an erasing operation is executed on the memory cells MCn andthe dummy cells DC, a preliminary writing operation (a weak writingoperation) for converging the threshold voltages of the respectivememory cells (the memory cells MCn and the dummy cells DC) within acertain range is executed on the respective memory cells (the memorycells MCn and the dummy cells DC). The number of times N of executingthe pre-program operation can be designated according to control datastored in, for example, a ROM fuse in a pre-program circuit 31. Thepre-program operation according to the present embodiment will now beexplained.

FIG. 16 is an explanatory diagram showing the concept of the pre-programoperation. The example shown in FIG. 16 assumes a nonvolatile memorywhich stores four-value data. By applying a pre-program voltage to eachmemory cell, the threshold voltage distribution Dp is converged within acertain range after the pre-program operation. By executing asimultaneous erasing operation on all the memory cells after this, it ispossible to optimize the erasing operation on the memory cells.

FIGS. 17A and 17B are diagrams explaining a pre-program operationaccording to a comparative example of the present embodiment. FIG. 17Ais a diagram showing a circuit configuration, and FIG. 17B is a diagramshowing a threshold voltage distribution after the pre-programoperation. In this comparative example, the same pre-program voltage Vp1(e.g., 10V) is applied to the word lines WLn and to the dummy word linesWLDS and WLDD as shown in FIG. 17A.

Select transistors STS and STD are connected to the memory cells (dummycells DC) at both ends of the memory string, respectively. Therefore,when the semiconductor device is miniaturized, the dummy cells DCreceive influence from the select transistors STS and STD, and hence thethreshold voltages of the dummy cells DC might become different from thethreshold voltages of the other memory cells MC0 to MC63. In the exampleshown in FIG. 17B, the threshold voltages of the dummy cells DC arehigher than the threshold voltages of the other memory cells MC0 toMC63. The phenomenon that the threshold voltages of the dummy cells DCbecome higher than the threshold voltages of the other memory cells MC0to MC 63 is observed after writing operations and erasing operationshave been repeatedly executed. Conversely, in an initial state beforewriting operations and erasing operations have been repeatedly executed,a phenomenon that the threshold voltages of the dummy cells DC are lowerthan the threshold voltages of the other memory cells MC0 to MC63 isobserved. Therefore, after a pre-program operation is executed, thethreshold voltages of the memory cells at both ends of the memory string(the dummy cells DC in the present embodiment) might be different fromthe threshold voltages of the other memory cells (the memory cells MC0to MC63 in the present embodiment). If such a condition occurs, it willbecome impossible to converge the threshold voltages of all the memorycells (the dummy cells DC and the memory cells MC0 to MC63) in thememory string within a certain range in an erasing operation to beexecuted later. As a result, it will become hard to execute an optimumerasing operation.

FIG. 18A and FIG. 18B are diagrams explaining a pre-program voltageaccording to the present embodiment. FIG. 18A is a diagram showing acircuit configuration, and FIG. 18B is a diagram showing a thresholdvoltage distribution after a pre-program operation. In the presentembodiment, as shown in FIG. 18A, a pre-program voltage Vp1 (e.g., 10V)is applied to the word lines WLn, and a pre-program voltage Vp2 (e.g.,9V) is applied to the dummy word lines WLDS and WLDD.

Hence, the memory cells at both ends of the memory string (the dummycells DC in the present embodiment) are applied with a pre-programvoltage different from a voltage applied to the other memory cells (thememory cells MC0 to MC63 in the present embodiment). Specifically, suchpre-program voltages are applied that the threshold voltages of thedummy cells DC become substantially equal to the threshold voltages ofthe memory cells MC0 to MC63. As a result, as shown in FIG. 18B, thethreshold voltages of all the memory cells in the memory string (thedummy cells DC and the memory cells MC0 to MC63) can becomesubstantially equal after the pre-program operation. That is, thethreshold voltages of all the memory cells in the memory string can beconverged within a certain range. As a result, in a subsequentsimultaneous erasing operation on all the memory cells, the erasingoperation can be executed securely.

As described above, after writing operations and erasing operations havebeen executed repeatedly, the threshold voltages of the memory cells atboth ends of the memory string (the dummy cells DC in the presentembodiment) tend to be higher than the threshold voltages of the othermemory cells (the memory cells MC0 to MC63 in the present embodiment).Conversely, in an initial state before writing operations and erasingoperations have been executed repeatedly, the threshold voltages of thememory cells at both ends of the memory string tend to be lower than thethreshold voltages of the other memory cells. That is, as the number oftimes writing/erasing operations have been executed increases, ease ofshift of the threshold voltages of the memory cells at both ends of thememory string gradually change. Therefore, it is preferable that thepre-program voltage can be varied according to the shift of thethreshold voltages of the memory cells at both ends of the memorystring.

Specifically, the total number of times writing operations and erasingoperations have been executed is counted (for example, a counter isprovided in the control circuit 3 or the pre-program circuit 31), andthe pre-program voltage is varied according to the counted number. Thatis, an optimum pre-program voltage is set according to the countednumber. Specifically, a relationship between the number of writingoperations and erasing operations counted from an initial state and anamount of shift of the threshold voltages from the initial state isderived in advance, and the relationship is set in a table in thecontrol circuit 3 or the pre-program circuit 31. Then, an optimumpre-program voltage is set with reference to the table.

Alternatively, data such as the number of times of loops (the number oftimes of step-ups) in a writing operation, the number of times of loops(the number of times of step-ups) in an erasing operation, etc. may bestored, and the pre-program voltage may be varied by determining thedegree of deterioration of the memory cells based on such data.

As shown in FIG. 15, after the pre-program operation, an erasing pulseapplying operation is executed on the memory cells MCn and the dummycells DC (step S112). That is, the same erasing pulse voltage is appliedto all the memory cells in the memory string (the memory cells MCn andthe dummy cells DC). In the present embodiment, an optimum erasingoperation can be executed since the threshold voltages of all the memorycells in the memory string (the memory cells MCn and the dummy cells DC)have been converged within a certain range.

After the erasing operation is executed, a verify operation is executedon the memory cells MCn and the dummy cells DC (step S113). Then, it isdetermined whether or not a certain condition is satisfied as a resultof the verify operation (step S114). Specifically, it is determinedwhether or not the erasing operation has been executed properly on allthe memory cells (the memory cells MCn and the dummy cells DC). When thecertain condition is not satisfied, the flow returns to step S112, andan erasing operation is executed again.

When the certain condition is satisfied, the erasing operation and aseries of operations following the erasing operation are completed.

As described above, according to the present embodiment, the memorycells at both ends of the memory string are applied with a pre-programvoltage different from the voltage applied to the other memory cells.Therefore, after the pre-program operation is executed, the thresholdvoltages of all the memory cells in the memory string can be convergedwithin a certain range. As a result, an optimum erasing operation can beexecuted, and a semiconductor device (nonvolatile semiconductor memory)having a high reliability can be obtained.

Modification Example 1 of Third Embodiment

FIG. 19 is a flowchart showing an operation according to a firstmodification example of the present embodiment. The basic operation isthe same as the operation shown in FIG. 15, and explanation about anymatters already described with reference to FIG. 15 will not beprovided.

In the present modification example, after a pre-program operation isexecuted, a verify operation is executed on the memory cells MCn and thedummy cells DC (step S121). Then, it is determined whether or not acertain condition is satisfied as a result of the verify operation.Specifically, it is determined whether or not the threshold voltages ofall the memory cells in the memory string (the dummy cells DC and thememory cells MC0 to MC63) have been converged within a certain range.When the certain condition is not satisfied, the flow returns to stepS111, and the pre-program operation is executed again.

The verify operation may be executed only on the memory cells at bothends of the memory string (the dummy cells DC in the presentembodiment). As can be understood from the description given above, thethreshold voltages of the memory cells at both ends of the memory stringgreatly shift. Therefore, even if a verify operation is executed only onthe memory cells at both ends of the memory string, it can be expectedthat a verify result having a certain degree of properness can beobtained. In this way, by executing the verify operation only on thememory cells at both ends of the memory string, it is possible toshorten the verify time.

When it is determined that the certain condition is satisfied as aresult of the verify operation after the pre-program operation, anerasing pulse applying operation is executed on the memory cells MCn andthe dummy cells DC (step S112). The following operations are the same asthe flowchart shown in FIG. 15.

Modification Example 2 of Third Embodiment

FIG. 20 is a flowchart showing an operation according to a secondmodification example of the present embodiment. The basic operation isthe same as the operation shown in FIG. 15 and FIG. 19, and explanationabout any matters already described with reference to FIG. 15 and FIG.19 will not be provided.

In the present modification example, before a pre-program operation isexecuted, a pre-pre-program verify operation is executed on the memorycells MCn and the dummy cells DC (step S131). Specifically, thethreshold voltages of the memory cells at both ends of the memory stringare measured before a pre-program operation is executed, so that thepre-program voltage may be changed according to the measurement result.That is, an optimum pre-program voltage is set according to themeasurement result. The pre-pre-program verify operation may be executedat different verify levels, to ensure a more appropriate determinationof the threshold voltages. That is, two or more times of pre-pre-programverify operations may be executed. The following operations are the sameas the flowchart shown in FIG. 19.

Modification Example 3 of Third Embodiment

FIG. 21 is a flowchart showing an operation according to a thirdmodification example of the present embodiment. The basic operation isthe same as the operation shown in FIG. 15, FIG. 19, and FIG. 20, andexplanation about any matters already described with reference to FIG.15, FIG. 19, and FIG. 20 will not be provided.

In the present modification example, before a pre-program operation isexecuted, a pre-pre-program verify operation is executed on the memorycells MCn and the dummy cells DC (step S131). Then, it is determinedwhether or not a certain condition is satisfied as a result of theverify operation, i.e., whether or not a pre-program operation need tobe executed (step S141). Specifically, it is determined whether or notthe threshold voltages of all the memory cells in the memory string (thedummy cells DC and the memory cells MCn) are converged within a certainrange. When it is determined that the certain condition is satisfied,the pre-program operation is not executed and the flow goes to stepS112. The operations following the pre-program operation (step S111) arethe same as the flowchart shown in FIG. 19.

In the present modification example, it is possible to prevent a writingstress and a time loss that would be caused by an unnecessarypre-program operation.

Modification Example 4 of Third Embodiment

FIG. 22 is a flowchart showing an operation according to a fourthmodification example of the present embodiment. The basic operation isthe same as the operation shown in FIG. 15, and explanation about anymatters already described with reference to FIG. 15 will not beprovided. The matters described in the first to third modificationexamples may be combined with the present example.

In the present modification example, after the processes up to step S114are executed in the same way as FIG. 15, a soft program pulse applyingoperation is executed on the memory cells MCn and the dummy cells DC(step S151). That is, after an erasing operation is executed on thememory cells MCn and the dummy cells DC, a preliminary writing operation(a weak writing operation) for converging the threshold voltages of therespective memory cells (the memory cells MCn and the dummy cells DC)within a certain narrower range is executed on the memory cells (thememory cells MCn and the dummy cells DC).

After the soft program operation is executed, a first soft programverify operation is executed on the memory cells MCn and the dummy cellsDC (step S152). Then, it is determined whether or not a certaincondition is satisfied as a result of the verify operation (step S153).Specifically, it is determined whether or not there is any string onwhich the soft program operation has not been executed properly. Whenthe certain condition is not satisfied, the flow returns to step S151and the soft program operation is executed again.

When it is determined that the certain condition is satisfied as aresult of the first soft program verify operation, a second soft programverify operation is executed on the memory cells MCn and the dummy cellsDC (step S154). Then, it is determined whether or not a certaincondition is satisfied as a result of the verify operation (step S155).Specifically, it is determined whether or not the number of bits onwhich the soft program operation has not been executed properly issmaller than a criterion value. When the certain condition is notsatisfied, the flow returns to step S112, and the erasing operation isexecuted again. When the certain condition is satisfied, the series ofoperations is completed.

In the embodiment described above, the memory cells at both ends of thememory string are dummy cells which are not used for an actual storageoperation. However, it is not necessary that they are dummy cells, butthey may be memory cells used for an actual storage operation. Also inthis case, the same effect as described above can be obtained.

In the embodiment described above, only the memory cells at the veryends of the memory string are applied with a pre-program voltagedifferent from the voltage to be applied to the other memory cells.However, a plurality of memory cells including the memory cells at thevery ends may be applied with a pre-program voltage different from thevoltage to be applied to the other memory cells. Furthermore, memorycells other than those at the very ends of the memory string may beapplied with a pre-program voltage different from the voltage to beapplied to the other memory cells. Generally, it is possible that atleast one memory cell in the memory string is applied with a pre-programvoltage different from a pre-program voltage to be applied to the othermemory cell in the memory string. In this case, it is possible to varythe pre-program voltage to be applied to such at least one nonvolatilememory cell.

[Others]

Though the embodiments of the invention have been explained, the presentinvention is not limited to these embodiments, but various alterations,additions, combinations, etc. can be made thereonto within the scope ofspirit of the invention. For example, the memory cell array may beconfigured with no dummy cells DC provided at the ends of the NAND cellunit 1. The explanation of the embodiments given above will apply tothis case of no dummy cells DC being provided, if the dummy cells DC areread as memory cells MC0 and MC63 and the dummy word lines WLDS and WLDDare read as word lines WL0 and WL63.

It is only necessary that the number of memory cells MCn connected inseries between the select transistors STD and STS should be a pluralnumber (a power of two), which is not limited to 64. In the aboveexplanation, the data stored in a memory cell is binary data orfour-value data, but it may be data of any other value (e.g.,eight-value data).

The present invention can also be applied to a case that there is adummy cell DC provided at only the side of either the drain-side selectgate line SGD or the source-side select gate line SGS of the NAND cellunit 1. Moreover, there may be not only a single dummy cell DC but twoor more dummy cells DC provided at each end of the NAND cell unit 1. Inthis case, the present invention may be applied to only the dummy cellsDC that adjoin the drain-side select gate line SGD and the source-sideselect gate line SGS or may be applied to all the dummy cells DC.

What is claimed is:
 1. A nonvolatile semiconductor memory device,comprising: a memory string including a plurality of nonvolatile memorycells for data storage which are capable of storing multi-bit dataconnected in series; a first dummy cell which is not used for datastorage provided at at least a first end of the memory string; selecttransistors connected to both ends of the memory string respectively;word lines connected to control gate electrodes of the nonvolatilememory cells for data storage; a first dummy word line connected to acontrol gate electrode of the first dummy cell; a bit line connected toa first end of the memory string; a source line connected to a secondend of the memory string; and a control circuit configured to perform anerasing operation by applying an erasing voltage to the nonvolatilememory cell, wherein before a pre-program operation before an erasingoperation to the nonvolatile memory cells, the control circuit performsa pre-preverify operation to the nonvolatile memory cells.